1. Field of Invention
The subject of the present invention is a vertical MOSFET transistor, in particular operating as a selector in nonvolatile memory devices.
The following description relates to a MOSFET transistor designed to operate as selector for a phase-change memory, without, however, being limited to this type of application. In particular, the MOSFET transistor according to the invention can be advantageously used also for other types of nonvolatile memories or other common-source-region applications.
2. Description of the Related Art
As is known, phase-change memories (PCMB) exploit the characteristics of materials that have the property of switching between two phases that have different electrical characteristics. For example, these materials can switch between an amorphous, disorderly phase and a crystalline or polycrystalline, orderly phase, and the two phases are associated to resistivities of markedly different values.
Currently, the alloys of group VI of the periodic table, such as Te or Se, referred to as calcogenides or calcogenic materials, may advantageously be used in phase-change memories. The currently most promising calcogenide is formed by a Ge, Sb, and Te alloy (Ge2Sb2Te5), which is currently widely used for storing information in overwritable disks.
In phase-change memories comprising calcogenic elements as storage elements, the memory cells are arranged in rows and columns, as illustrated in FIG. 1. The memory array 1 of FIG. 1 comprises a plurality of memory cells 2, arranged in the crossing points between rows 6 (also referred to as wordlines) and columns 5 (also referred to as bitlines) and each of which includes a memory element 3 of a phase-change type and a selection element 4.
In each memory cell 2, the memory element 3 has a first terminal connected to an own wordline 6 and a second terminal connected to a first conduction terminal of an own selection element 4. The selection element 4 has a second terminal connected to a bitline 5. In another solution, the memory element 3 and the selection element 4 of each cell 2 can be exchanged.
Currently, as selection elements bipolar transistors, planar MOS transistors, or diodes have been proposed.
A method of manufacturing a phase-change memory device including a diode as a selection element is described in US 200310219924. Another method for obtaining a memory comprising a bipolar transistor as a selection element is described in.
Phase-change memories using selectors made as bipolar components have the problem that they dissipate even when they are in the non-selected or off state thereby, as a whole, giving rise to a non-negligible consumption. Phase-change memories that use selectors made with planar MOS technology have the problem that they occupy a non-negligible area.